Self-contained partial database backups

ABSTRACT

Methods and computer readable media for restoring a database. In a method, a backup of a subset of a database is created, where the subset includes a filegroup and database metadata describing the filegroup. The database is then restored using the database subset. In a computer-readable medium, a first data field contains data representing a database subset having at least one of a plurality of filegroups. A second data field contains data representing database metadata that describes the filegroup. In another method, archived database metadata and an archived filegroup that is one of a plurality of filegroups is accessed, and the filegroup is brought online using the database metadata.

FIELD OF THE INVENTION

The present invention relates generally to database systems and, moreparticularly, to database backup and restoration.

BACKGROUND OF THE INVENTION

Databases are used in a variety of contexts and often contain largequantities of valuable information. For example, databases may be usedin commercial applications, such as electronic commerce, to processcustomer orders for merchandise, to maintain customer financialinformation and so forth. Periodically, information contained within adatabase may be backed up to a non-volatile storage medium to preservethe information in the event the database goes offline in anuncontrolled manner, such as due to a power failure or system error.Typically, it is important to bring such a database online as quickly aspossible so as to reduce errors and, potentially, lost business andrevenue. For example, an operator of a database that is used inelectronic commerce will want the database to come back online quicklyso the database can continue to manage orders and thereby generaterevenue for the operator.

Conventionally, restoration of a database is accomplished by backing upall of the database's filegroups to some form of non-volatile storage.After a fault occurs and the database goes offline, the database can bebrought back online by restoring all of the filegroups from storage aswell as any database metadata that is used to access the informationcontained in each filegroup. Because all filegroups within such adatabase must be restored before any access to the database ispermitted, there may be a long delay before the database is brought backonline. In the case of very large databases and/or databases that have alarge number of filegroups, the delay may be particularly pronounced.Conventionally, the requirement that all database filegroups be restoredprior to bringing a database back online cannot be waived even insituations where the database's most critical information is stored onlyin a few, or even one, filegroup. Thus, a conventional method forrestoring a database may delay bringing a database back online whilesuch a method restores relatively unimportant filegroups. Furthermore,the requirement that all filegroups be restored prior to bringing adatabase online necessitates that each filegroup be backed up in itsentirety to enable each such filegroup to be restored.

Accordingly, there is a need for a mechanism that enables a database toreturn to an online status without having fully restored all of itsfilegroups. The present invention satisfies this need.

SUMMARY OF THE INVENTION

In view of the foregoing shortcomings and drawbacks, the invention isdirected to methods and computer readable media for restoring adatabase. In one such method, a backup of a subset of a database iscreated, where the subset includes a filegroup and database metadatadescribing the filegroup. The database is then restored using thedatabase subset. In a computer-readable medium, a first data fieldcontains data representing a database subset having at least one of aplurality of filegroups. A second data field contains data representingdatabase metadata that describes the filegroup. In another method,archived database metadata and an archived filegroup that is one of aplurality of filegroups is accessed, and the filegroup is brought onlineusing the database metadata.

Additional features and advantages of the invention will be madeapparent from the following detailed description of illustrativeembodiments that proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofpreferred embodiments, is better understood when read in conjunctionwith the appended drawings. For the purpose of illustrating theinvention, there is shown in the drawings example constructions of theinvention; however, the invention is not limited to the specific methodsand instrumentalities disclosed. In the drawings:

FIG. 1 is a diagram illustrating an example computing environment inwhich aspects of the invention may be implemented;

FIG. 2 is a diagram illustrating an example computer network in whichaspects of the invention may be incorporated;

FIG. 3A is a block diagram illustrating an example database in whichaspects of the invention may be implemented;

FIG. 3B is a block diagram illustrating an example restored databaseaccording to an embodiment of the invention; and

FIG. 4 is a flowchart illustrating an example method of restoring adatabase according to an embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The subject matter of the present invention is described withspecificity to meet statutory requirements. However, the descriptionitself is not intended to limit the scope of this patent. Rather, theinventors have contemplated that the claimed subject matter might alsobe embodied in other ways, to include different steps or elementssimilar to the ones described in this document, in conjunction withother present or future technologies. Moreover, although the term “step”may be used herein to connote different aspects of methods employed, theterm should not be interpreted as implying any particular order among orbetween various steps herein disclosed unless and except when the orderof individual steps is explicitly described.

Example Computing Environment

FIG. 1 illustrates an example of a suitable computing system environment100 on which the invention may be implemented. The computing systemenvironment 100 is only one example of a suitable computing environmentand is not intended to suggest any limitation as to the scope of use orfunctionality of the invention. Neither should the computing environment100 be interpreted as having any dependency or requirement relating toany one or combination of components illustrated in the exampleoperating environment 100.

The invention is operational with numerous other general purpose orspecial purpose computing system environments or configurations.Examples of well known computing systems, environments, and/orconfigurations that may be suitable for use with the invention include,but are not limited to, personal computers, server computers, hand-heldor laptop devices, multiprocessor systems, microprocessor-based systems,set top boxes, programmable consumer electronics, network PCs,minicomputers, mainframe computers, distributed computing environmentsthat include any of the above systems or devices, and the like.

The invention may be described in the general context ofcomputer-executable instructions, such as program modules, beingexecuted by a computer. Generally, program modules include routines,programs, objects, components, data structures, etc., that performparticular tasks or implement particular abstract data types. Typicallythe functionality of the program modules may be combined or distributedas desired in various embodiments. The invention may also be practicedin distributed computing environments where tasks are performed byremote processing devices that are linked through a communicationsnetwork. In a distributed computing environment, program modules may belocated in both local and remote computer storage media including memorystorage devices.

With reference to FIG. 1, an example system for implementing theinvention includes a general purpose computing device in the form of acomputer 110. Components of computer 110 may include, but are notlimited to, a processing unit 120, a system memory 130, and a system bus121 that couples various system components including the system memoryto the processing unit 120. The system bus 121 may be any of severaltypes of bus structures including a memory bus or memory controller, aperipheral bus, and a local bus using any of a variety of busarchitectures. By way of example, and not limitation, such architecturesinclude Industry Standard Architecture (ISA) bus, Micro ChannelArchitecture (MCA) bus, Enhanced ISA (EISA) bus, Video ElectronicsStandards Association (VESA) local bus, and Peripheral ComponentInterconnect (PCI) bus also known as Mezzanine bus.

Computer 110 typically includes a variety of computer readable media.Computer readable media can be any available media that can be accessedby computer 110 and includes both volatile and nonvolatile media,removable and non-removable media. By way of example, and notlimitation, computer readable media may comprise computer storage mediaand communication media. Computer storage media includes both volatileand nonvolatile, removable and non-removable media implemented in anymethod or technology for storage of information such as computerreadable instructions, data structures, program modules or other data.Computer storage media includes, but is not limited to, RAM, ROM,EEPROM, flash memory or other memory technology, CD-ROM, digitalversatile disks (DVD) or other optical disk storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which can be used to store the desired informationand which can accessed by computer 110. Communication media typicallyembodies computer readable instructions, data structures, programmodules or other data in a modulated data signal such as a carrier waveor other transport mechanism and includes any information deliverymedia. The term “modulated data signal” means a signal that has one ormore of its characteristics set or changed in such a manner as to encodeinformation in the signal. By way of example, and not limitation,communication media includes wired media such as a wired network ordirect-wired connection, and wireless media such as acoustic, RF,infrared and other wireless media. Combinations of the any of the aboveshould also be included within the scope of computer readable media.

The system memory 130 includes computer storage media in the form ofvolatile and/or nonvolatile memory such as read-only memory (ROM) 131and random access memory (RAM) 132. A basic input/output system 133(BIOS), containing the basic routines that help to transfer informationbetween elements within computer 110, such as during start-up, istypically stored in ROM 131. RAM 132 typically contains data and/orprogram modules that are immediately accessible to and/or presentlybeing operated on by processing unit 120. By way of example, and notlimitation, FIG. 1 illustrates operating system 134, applicationprograms 135, other program modules 136 and program data 137.

The computer 110 may also include other removable/non-removable,volatile/nonvolatile computer storage media. By way of example only,FIG. 1 illustrates a hard disk drive 140 that reads from or writes tonon-removable, nonvolatile magnetic media, a magnetic disk drive 151that reads from or writes to a removable, nonvolatile magnetic disk 152,and an optical disk drive 155 that reads from or writes to a removable,nonvolatile optical disk 156 such as a CD ROM or other optical media.Other removable/non-removable, volatile/nonvolatile computer storagemedia that can be used in the example operating environment include, butare not limited to, magnetic tape cassettes, flash memory cards, digitalversatile disks, digital video tape, solid state RAM, solid state ROM,and the like. The hard disk drive 141 is typically connected to thesystem bus 121 through a non-removable memory interface such asinterface 140, and magnetic disk drive 151 and optical disk drive 155are typically connected to the system bus 121 by a removable memoryinterface, such as interface 150.

The drives and their associated computer storage media discussed aboveand illustrated in FIG. 1, provide storage of computer readableinstructions, data structures, program modules and other data for thecomputer 110. In FIG. 1, for example, hard disk drive 141 is illustratedas storing operating system 144, application programs 145, other programmodules 146 and program data 147. Note that these components can eitherbe the same as or different from operating system 134, applicationprograms 135, other program modules 136, and program data 137. Operatingsystem 144, application programs 145, other program modules 146 andprogram data 147 are given different numbers here to illustrate that, ata minimum, they are different copies. A user may enter commands andinformation into the computer 20 through input devices such as akeyboard 162 and pointing device 161, commonly referred to as a mouse,trackball or touch pad. Other input devices (not shown) may include amicrophone, joystick, game pad, satellite dish, scanner, or the like.These and other input devices are often connected to the processing unit120 through a user input interface 160 that is coupled to the systembus, but may be connected by other interface and bus structures, such asa parallel port, game port or a universal serial bus (USB). A monitor191 or other type of display device is also connected to the system bus121 via an interface, such as a video interface 190. In addition to themonitor, computers may also include other peripheral output devices suchas speakers 197 and printer 196, which may be connected through anoutput peripheral interface 190.

The computer 110 may operate in a networked environment using logicalconnections to one or more remote computers, such as a remote computer180. The remote computer 180 may be a personal computer, a server, arouter, a network PC, a peer device or other common network node, andtypically includes many or all of the elements described above relativeto the computer 110, although only a memory storage device 181 has beenillustrated in FIG. 1. The logical connections depicted in FIG. 1include a local area network (LAN) 171 and a wide area network (WAN)173, but may also include other networks. Such networking environmentsare commonplace in offices, enterprise-wide computer networks, intranetsand the Internet.

When used in a LAN networking environment, the computer 110 is connectedto the LAN 171 through a network interface or adapter 170. When used ina WAN networking environment, the computer 110 typically includes amodem 172 or other means for establishing communications over the WAN173, such as the Internet. The modem 172, which may be internal orexternal, may be connected to the system bus 121 via the user inputinterface 160, or other appropriate mechanism. In a networkedenvironment, program modules depicted relative to the computer 110, orportions thereof, may be stored in the remote memory storage device. Byway of example, and not limitation, FIG. 1 illustrates remoteapplication programs 185 as residing on memory device 181. It will beappreciated that the network connections shown are exemplary and othermeans of establishing a communications link between the computers may beused.

Example Distributed Computing Frameworks or Architectures

Various distributed computing frameworks have been and are beingdeveloped in light of the convergence of personal computing and theInternet. Individuals and business users alike are provided with aseamlessly interoperable and web-enabled interface for applications andcomputing devices, making computing activities increasingly web browseror network-oriented.

For example, MICROSOFT®'s .NET platform includes servers, building-blockservices, such as web-based data storage, and downloadable devicesoftware. Generally speaking, the .NET platform provides (1) the abilityto make the entire range of computing devices work together and to haveuser information automatically updated and synchronized on all of them,(2) increased interactive capability for web sites, enabled by greateruse of XML rather than HTML, (3) online services that feature customizedaccess and delivery of products and services to the user from a centralstarting point for the management of various applications, such ase-mail, for example, or software, such as Office NET, (4) centralizeddata storage, which will increase efficiency and ease of access toinformation, as well as synchronization of information among users anddevices, (5) the ability to integrate various communications media, suchas e-mail, faxes, and telephones, (6) for developers, the ability tocreate reusable modules, thereby increasing productivity and reducingthe number of programming errors, and (7) many other cross-platformintegration features as well.

While example embodiments herein are described in connection withsoftware residing on a computing device, one or more portions of theinvention may also be implemented via an operating system, API, ormiddleware software between a coprocessor and requesting object, suchthat services may be performed by, supported in, or accessed via all ofNET's languages and services, and in other distributed computingframeworks as well.

Network Environment

FIG. 2 illustrates an example network environment in which the presentinvention may be employed. Of course, actual network and databaseenvironments may be arranged in a variety of configurations; however,the example environment shown here provides a framework forunderstanding the type of environment in which an embodiment mayoperate.

The example network may include one or more client computers 200 a, aserver computer 200 b, data source computers 200 c, and/or databases270, 272 a, and 272 b. The client computers 200 a and the data sourcecomputers 200 c may be in electronic communication with the servercomputer 200 b by way of the communications network 280 (e.g., anintranet, the Internet or the like). The client computers 200 a and datasource computers 200 c may be connected to the communications network byway of communications interfaces 282. The communications interfaces 282can be any type of communications interfaces such as Ethernetconnections, modem connections, wireless connections and so on.

The server computer 200 b may provide management of the database 270 byway of database server system software such as MICROSOFT®'s SQL SERVERor the like. As such, server 200 b may act as a storehouse of data froma variety of data sources and provides that data to a variety of dataconsumers.

In the example network environment of FIG. 2, a data source may beprovided by data source computer 200 c. Data source computer 200 c maycommunicate data to server computer 200 b via communications network280, which may be a LAN, WAN, Intranet, Internet, or the like. Datasource computer 200 c may store data locally in database 272 a, whichmay be database server or the like. The data provided by data source 200c can be combined and stored in a large database such as a datawarehouse maintained by server 200 b.

Client computers 200 a that desire to use the data stored by servercomputer 200 b can access the database 270 via communications network280. Client computers 200 a access the data by way of, for example, aform, a query, etc. It will be appreciated that any configuration ofcomputers is equally compatible with an embodiment of the presentinvention.

Example Embodiments

In the discussion that follows, it is assumed that one skilled in theart has familiarity with the syntax of databases, such as the syntaxused in SQL SERVER and the like, and therefore details relating to suchsyntax are omitted herein for purposes of clarity. In addition, whilethe examples provided herein are described in connection with SQL SERVERsyntax, it will be appreciated that an embodiment is equally applicableto any type of database program, syntax, application or the like.Furthermore, while some examples herein refer to databases used inconnection with electronic commerce, it should be appreciated that anembodiment is equally applicable to a database that is used inconnection with any field of endeavor for any purpose.

As discussed above, conventional database backup systems require thatall filegroups and metadata of a database be backed up and restoredprior to bringing a database online. An embodiment provides a mechanismfor bringing a subset of an entire database online so as to decrease thetime required to return a database to service. The subset may, in anembodiment, only contain database metadata and a filegroup. It will beappreciated from the discussion that follows that additional filegroupsmay be brought back online after the database itself, so that a fulldatabase restoration is also contemplated by an embodiment.

For purposes of explanation, FIG. 3A illustrates an example database inwhich aspects of the invention may be implemented. Referring now to FIG.3A, database 300 may include read-write filegroups 310-315 and read-onlyfilegroups 320-325. It will be appreciated that read-write filegroups310-315 may include any type of files, data or the like that permit bothreading and modification of their contents. Likewise, read-onlyfilegroups 320-325 may be any type of files, data or the like that onlypermit reading of their contents. In addition to filegroups 310-325,database 300 may include database metadata 330, which may be any type ofdata that may be used to identify and access the various filegroups310-325.

Database 300 may also include log files 340. Log files 340 may includeinformation that indicates a modification to a filegroup 310-325, thetime at which a filegroup 310-325 was last archived (i.e., saved to anon-volatile storage medium), the time at which a read-write filegroupwas made read-only or vice-versa (if applicable), and so forth. As maybe appreciated, therefore, log files 340 may include any type ofinformation that describes filegroup 310-325, an operation performed onsuch filegroups 310-325 or the like. For example, log files 340 mayinclude a log sequence number that indicates a time at which a filegroup310-325 was last archived. The log sequence number may, alternatively,include a number or other identifier that simply increases every timefilegroup 310-325 is archived. As may be appreciated, the log sequencenumber may contain any type of information to indicate how recently aparticular filegroup 310-325 was archived.

As noted above, an embodiment permits a database 300 to resumeoperations in a partially-restored state. An example of such apartially-restored state is illustrated in FIG. 3B, which illustratesonly the components that are online. Referring now to FIG. 3B, database300 can be seen, having log files 340, database metadata 330 andread-write filegroups 310-315. Because of their absence, it can be seenin FIG. 3B that read-only filegroups 320-325 are not online. It will beappreciated that the illustration of FIG. 3B is merely for purposes ofexplanation and that, as will be discussed below, the configuration ofrestored database 300 may take any form, and may include any number ortype of read-write filegroups 310-315 and/or read-only filegroups320-325 (not shown in FIG. 3B for clarity).

Now that an example database 300 and restored database 300 has beendescribed in connection with FIGS. 3A-B, an example method of restoringa database according to an embodiment will now be discussed inconnection with FIG. 4. Turning now to FIG. 4, example method 400according to an embodiment is illustrated.

At step 401, a filegroup subset of a database is selected. It will beappreciated that any criteria may be used for such selection. Forexample, the minimum number of filegroups that are necessary for adatabase to perform its most important function(s) may be selected sothe database may be brought back online as quickly as possible. Inanother embodiment, a database user may be willing to trade-off acertain amount of time to bring a database online in exchange for agreater number of filegroups, even if such filegroups are not absolutelyessential. In yet another embodiment, only read-write filegroups areselected. Thus, it can be seen that any number or type of databasefilegroups may be selected in connection with step 401, and that thefilegroups so selected may, in an embodiment, ultimately be used torestore the database in step 409 below.

At step 403, a subset of the database, including the filegroup(s)selected in connection with step 401 and database metadata, may bebacked up (i.e., archived or the like) to some form of non-volatilestorage, such as a hard drive, CD-ROM, DVD and so forth. Step 403 may beperformed on-demand, periodically according to some predeterminedschedule, or the like. As should be known to one of skill in the art,any syntax that is appropriate to the database may be used to performstep 403. An example of such syntax may take the form:

BACKUP DATABASE sampleDb READ_WRITE_FILEGROUPS

In the above example, the syntax READ_WRITE_FILEGROUPS is used to backup only the database filegroups of a database called sampleDb that areread-write. Thus, it will be appreciated that the above example syntaxmay be used when the selection performed in connection with step 401 wasfor all read-write filegroups.

It will also be appreciated that in some embodiments, steps 401 and 403may be combined into a single step that either backs up the entiredatabase or that backs up a subset of the database that is larger, orotherwise different, than the subset of the database that will be usedin connection with step 409 to restore the database. As will bediscussed in connection with step 409 below, an embodiment can selectappropriate portions of the database to restore from amongst archiveddata.

At optional step 405, a log sequence number may assigned to eachfilegroup to indicate a time at which the filegroup was backed up. Thus,if more than one copy of a filegroup is available, a system on which thedatabase is running could determine the most up-to-date version.Alternatively, and as noted above, the log sequence number may be usedto, for example, record the last time at which a filegroup was maderead-only.

At some point, signified by step 407, the database goes offline orotherwise encounters a fault that requires the database to be restored.As may be appreciated, such a fault may be due to power loss orfluctuation, software error or crash, user error, etc. The database mayalso be brought offline in a controlled manner because of a plannedoutage or due to an emergency, for example.

At step 409, the database is restored using the stored database metadataand the filegroup subset that was saved in connection with step 403. Anexample syntax that may be used to accomplish step 409 may take theform:

RESTORE DATABASE sampleDb WITH PARTIAL

In the above example, the WITH PARTIAL syntax can, according to anembodiment, be used to partially restore a database. This is as opposedto the conventional use of the WITH PARTIAL syntax, which permitted apartial restore from only a full backup and rendered permanently offlineany filegroups that were not part of the partial restore. Furthermore,and as was noted above, the filegroup subset that is used to restore thedatabase in step 409 may differ from the subset that was backed up inconnection with step 403, above. Thus, in an embodiment any combinationof read-write and read-only filegroups may be a part of apartially-restored database. For example, an electronic commerceapplication of a database may require two read-write filegroups and oneread-only filegroup to be able to handle incoming customer orders. Thus,such a subset of an entire database may be restored. Therefore, anembodiment allows a user or the like to select one or more filegroups torestore from amongst any number of archived filegroups, including acomplete backup of the database.

At step 411, the database may be brought back online in apartially-restored status. At optional step 413, filegroups that werenot restored in connection with step 409 and were therefore not broughtback online at step 411 may be restored and ultimately brought backonline. In one embodiment, the subset of database filegroups may berestored to the same system on which the database was running prior togoing offline. In such an embodiment, the filegroup(s) that were notrestored may be restored at a later time, either periodically,as-needed, or according to some other criteria. When bringing such afilegroup online, a determination may be made as to whether any of apossible plurality of stored filegroups are current, as determined bythe log sequence number. For example, such a system may have a copy ofthe filegroup in memory, as well as an archived copy of the samefilegroup. In such a case, the system may use the log sequence number todetermine the most recent version of the filegroup, and then bring themost recent version online.

In another embodiment, the subset of database filegroups may be restoredto a different system from that which the database was running prior togoing offline. As was the case in the embodiment discussed above, thefilegroup(s) that were not restored may be restored at a later time,either periodically, as-needed, or according to some other criteria. Asmay be appreciated, however, the different system that is going to runthe database may not have any saved filegroups, but the unrestoredfilegroups may be accessible by way of their database metadata. Ineither case, the restoration of additional filegroups may be performedwhile the database is offline or online.

While the present invention has been described in connection with theembodiments of the various figures, it is to be understood that othersimilar embodiments may be used or modifications and additions may bemade to the described embodiment for performing the same function of thepresent invention without deviating therefrom. Therefore, the presentinvention should not be limited to any single embodiment, but rathershould be construed in breadth and scope in accordance with the appendedclaims.

1. A method of restoring a database, comprising: selecting, for backup,at least two filegroups comprising a subset of a database, the databasecomprising a plurality of different filegroups, each filegroupcomprising a discrete set of files or data, the subset comprising the atleast two filegroups and database metadata describing the at least twofilegroups, wherein the selection is based upon user-specified criteriarelated to the at least two filegroups and wherein the at least twoselected filegroups include at least one filegroup required to partiallyrestore the database and at least one filegroup that is not essential topartially restore the database, wherein the at least two selectedfilegroups are selected for backup based on a user-selected amount oftime required to restore the at least two filegroups; creating a backupof the subset of the database; restoring the database subset andoperating the database with the subset, wherein the database is able toresume operations in a partially-restored state; and restoring one ormore additional filegroups after the database has been operating withthe database subset.
 2. The method of claim 1, wherein the at least twofilegroups comprises a plurality of read-write files.
 3. The method ofclaim 1, wherein creating a backup of the subset of the databasecomprises saving the database metadata and the at least two filegroupsto a non-volatile storage medium.
 4. The method of claim 1, wherein theone or more additional filegroups comprises a plurality of read-onlyfiles.
 5. The method of claim 4, further comprising assigning a logsequence number to the one or more additional filegroups and creating abackup of the one or more additional filegroups.
 6. The method of claim5, wherein the log sequence number corresponds to a time at which theone or more additional filegroups are made read-only.
 7. A method forrestoring a database, comprising; archiving at least two filegroups of adatabase having a plurality of filegroups, each filegroup comprising adiscrete set of files or data, wherein each of the at least twofilegroups is selected based upon user-specified criteria related to theat least two filegroups, wherein the at least two selected filegroupsincludes at least one filegroup required to partially-restore thedatabase and at least one filegroup that is not essential to partiallyrestore the database, wherein the at least two selected filegroups areselected based on a user-selected amount of time required to restore theat least two filegroups; accessing archived database metadata and the atleast two archived filegroups; bringing the at least two filegroupsonline using the database metadata and operating the database with theat least two filegroups, wherein the database is able to resumeoperations in a partially-restored state; and bringing additionalarchived filegroups online after the database has resumed operationswith the at least two filegroups.
 8. The method of claim 7, furthercomprising: accessing one of the additional archived filegroups and alog sequence number associated with the accessed filegroup; and bringingthe accessed filegroup online if the associated log sequence numberindicates that the accessed filegroup is the most recent version of theaccessed filegroup.
 9. The method of claim 8, wherein the at least twofilegroups and the additional archived filegroups are selected from aplurality of archived database filegroups.